(a) Technical Field
The present invention relates to a multi-layer mold coating material for use in forming an ultra-high strength steel sheet. More particularly, the present invention relates to a multi-layer mold coating material for forming an ultra-high strength steel sheet that provides heat resistance, abrasion resistance, and low friction characteristics to the surface of the mold.
(b) Background Art
Steel sheet may be formed by a variety of processes such as, for example, deep drawing, incremental sheet forming, punching, rolling, stamping, etc. These processes typically require a tool piece (e.g., a mold, a die, punch, etc.) in order to form the steel sheet. Such tool pieces are subject to extreme wear during the metal working process. Conventionally, such tool pieces are treated or coated (e.g., by chemical vapor deposition (CVD), physical vapor deposition (PVD), quenching, tempering, diffusion hardening, etc.) so as to increase their longevity for use in the metal forming process.
Conventional art coating materials for a mold for forming a steel sheet for vehicles typically include, for example, Toyota Diffusion (TD), TiAlN, and AlTiCrN+MoS2. These coating materials are frequently used for molds used to form steel sheet with a tensile strength of 800 MPa or less, and provide a tool piece (e.g., mold/die) lifespan of about 70,000 press strokes.
Recent trends for steel sheet development are toward high strength/light weight thin-wall steel (e.g., for vehicle safety applications). Consequently, typical mold coating materials are deficient in durability and abrasion resistance when used to form steel sheets with a tensile strength of 980 MPa or more. In other words, a major disadvantage of conventional coating materials is that they provide a tool piece lifespan of only about 50,000 press strokes or less, after which the coating layer is exfoliated from the surface of the tool piece (e.g., the mold). For example, a mold with a typical TD coating layer has a lifespan of only about 30,000 strokes while manually modifying a deformation upon heat treatment.
Another disadvantage of conventional coating materials is that many require the use of a lubricant during the steel sheet forming process. For example, coating materials such as TiAlN and Al TiCrN+MoS2 typically require the use of a drawing oil due to deficiency of durability of the coated tool piece. While the use of a lubricant allows friction and abrasion to be temporarily reduced, the drawing oil may introduce impurities into the steel sheet that may negatively affect a subsequent process, e.g., a welding process, causing a reduction in the quality of the molded products.
Accordingly, there is a need for tool piece coating materials with abrasion resistance, heat resistance, fatigue resistance, high lubrication, and low friction characteristics, for molding steel sheet with a tensile strength of 980 MPa or more.